spliting on ":"

[s99999999s2003]

hi

i have a file with

xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx:yyy

i wanna split on ":" and get all the "yyy" and print the whole line out
so i did

print line.split(":")[-1]

but line 4 and 5 are not printed as there is no ":" to split. It should
print a "blank" at least.
how to print out lines 4 and 5 ?
eg output is

yyy
yyy
yyy
yyy

thanks

[Duncan Booth]

s9************@yahoo.com wrote:

print line.split(":")[-1]

but line 4 and 5 are not printed as there is no ":" to split. It should
print a "blank" at least.
how to print out lines 4 and 5 ?
eg output is

yyy
yyy
yyy
yyy

if ":" in line:
print line.split(":")[-1]
else:
print

what's the problem?

[Kent Johnson]

s9************@yahoo.com wrote:

hi

i have a file with

xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx:yyy

i wanna split on ":" and get all the "yyy" and print the whole line out
so i did

print line.split(":")[-1]

but line 4 and 5 are not printed as there is no ":" to split. It should
print a "blank" at least.
how to print out lines 4 and 5 ?
eg output is

yyy
yyy
yyy
yyy

That's not what I get:

In [2]: data='''xxx.xxx.xxx.xxx:yyy
...: xxx.xxx.xxx.xxx:yyy
...: xxx.xxx.xxx.xxx:yyy
...: xxx.xxx.xxx.xxx
...: xxx.xxx.xxx.xxx
...: xxx.xxx.xxx.xxx:yyy'''.splitlines()

In [3]: for line in data:
...: print line.split(':')[-1]
...:
...:
yyy
yyy
yyy
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx
yyy

Kent

[Peter Hansen]

Cyril Bazin wrote:

Your file looks like a list of IP adresses.
You can use the urllib and urllib2 modules to parse IP adresses.

import urllib2
for line in open("fileName.txt"):
addr, port = urllib2.splitport(line)
print (port != None) and '' or port

Is this what you want to happen when port is None?

port = None
print (port != None) and '' or port

None
I think you're getting caught by the classic and/or trap in Python,
trying to avoid using a simple if statement.

By the way, equality comparison with None is generally a bad idea as
well, so even if the above worked it should be (port is not None) rather
than (port != None).

-Peter

[Petr Jakes]

> I think you're getting caught by the classic and/or trap in Python,

trying to avoid using a simple if statement.

What do you mean by "the classic and/or trap"? Can you give an example
please?

Petr Jakes

[Peter Hansen]

Cyril Bazin wrote:

Ok, ok, there was a mistake in the code.
(Of course, it was done on prupose in order to verify if everybody is
aware ;-)
I don't know why it is preferable to compare an object to the object
"None" using "is not".
"==" is, IMHO, more simple. Simple is better than complex.. So I use "==".
The archives could tell you more, but basically on is usually interested
in *identity* with a singleton object (None), not in whether the object
on is examining happens to compare equal. A custom object could be
designed to compare equal to None in certain cases, even though it *is
not* None, leading to the "== None" approach being defective code.

In the specific code in question, it won't make any differences, but I
pointed it out to help folks who don't know this to start developing the
safer habit, which is always to use "is" and "is not" with None (or,
generally, with other singletons).
The correct program is:

import urllib2
for line in open("fileName.txt"):
addr, port = urllib2.splitport (line)
print (port == None) and '' or port
Nope, sorry... the point is that the "and/or" pseudo-ternary operator is
inherently flawed if the operand after "and" evaluates False. Check
this out:

port = None
print (port == None) and '' or port None print (port != None) and '' or port None

and even:
print (port is None) and '' or port

None

So the bug isn't in using "!=" instead of "==", or using equality
instead of identity comparison, it is in trying to use the and/or
expression for a purpose it wasn't intended for.
or

import urllib2
for line in open("fileName.txt"):
addr, port = urllib2.splitport(line)
if port == None:
print ''
else:
print port

That should work nicely... and it's more readable too!

Note that in Python 2.5 you should be able to write this as

print '' if port is None else port
or print '' if (port is None) else port

but it's quite arguable whether that is better than the simple if/else
statement in this case.

-Peter

[Peter Hansen]

Petr Jakes wrote:

I think you're getting caught by the classic and/or trap in Python,
trying to avoid using a simple if statement.

What do you mean by "the classic and/or trap"? Can you give an example
please?

Sure, see my subsequent reply to Cyril, elsewhere in this thread.

(In summary, and/or is defective when used as a ternary operator if the
expression after "and" evaluates False. The list archives can tell you
more.)

-Peter

[Steven D'Aprano]

On Sat, 04 Mar 2006 08:54:33 -0800, s99999999s2003 wrote:

hi

i have a file with

xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx:yyy
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx:yyy

i wanna split on ":" and get all the "yyy" and print the whole line out
so i did

print line.split(":")[-1]

but line 4 and 5 are not printed as there is no ":" to split. It should
print a "blank" at least.
how to print out lines 4 and 5 ?
eg output is

yyy
yyy
yyy
yyy

There are a few ways of handling this problem. It depends on what you want
to do if your input string looks like this "abc:y:z": should your code
print "y", "z" or even "y:z"?

# look before you leap
line = "abc:y:z"
if ":" in line:
print line.split(":")[-1] # prints "z"
print line.split(":")[1] # prints "y"
print line.split(":", 1)[-1] # prints "y:z"
# if your line only has one colon, the above three lines
# should all print the same thing
else:
print ""

# look before you leap again
L = line.split(":")
if len(L) > 1:
print L[1]
else:
print ""

# use a sentinel value
print (line + ":").split(":", 1)[1][:-1]
--
Steven.

[fileexit]

yyy
yyy
yyy
xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx

of course you will get this result...

inside the loop, when line="xxx.xxx.xxx.xxx:yyy"
line.split(":") will give a list ["xxx.xxx.xxx.xxx", "yyy"], and
element -1 will be "yyy"

but when line="xxx.xxx.xxx.xxx"
line.split(":") will give a list ["xxx.xxx.xxx.xxx"], and element -1
will be "xxx.xxx.xxx.xxx"

So the result is very very normal

[Bryan Olson]

Peter Hansen wrote:

The archives could tell you more, but basically on is usually interested
in *identity* with a singleton object (None), not in whether the object
on is examining happens to compare equal. A custom object could be
designed to compare equal to None in certain cases, even though it *is
not* None, leading to the "== None" approach being defective code.
But if a custom class allows instances to compare as equal to None,
we might reasonably expect the programmers had a reason. There's not
much anyone can do with None besides passing it around and comparing
it by value or identity. Insisting on 'is' rather than '==' will break
whatever polymorphism such a custom object was trying to achieve.

In the specific code in question, it won't make any differences, but I
pointed it out to help folks who don't know this to start developing the
safer habit, which is always to use "is" and "is not" with None (or,
generally, with other singletons).

Hmmm... To make my code safer, I'm thinking I should replace doc strings
that say "if bluf is Null" with "if blurf compares equal to Null".
--
--Bryan

[Steve Holden]

Bryan Olson wrote:

Peter Hansen wrote:

The archives could tell you more, but basically on is usually interested
in *identity* with a singleton object (None), not in whether the object
on is examining happens to compare equal. A custom object could be
designed to compare equal to None in certain cases, even though it *is
not* None, leading to the "== None" approach being defective code.

But if a custom class allows instances to compare as equal to None,
we might reasonably expect the programmers had a reason. There's not
much anyone can do with None besides passing it around and comparing
it by value or identity. Insisting on 'is' rather than '==' will break
whatever polymorphism such a custom object was trying to achieve.

That's all very well, but completely hypothetical. What's the use case
for this theoretical object? Why should anything that isn't None compare
equal with it?

None is explicitly defined as a type with a single instance, so
comparing equal to None without *being* None seems like a *very* bad idea.

In the specific code in question, it won't make any differences, but I
pointed it out to help folks who don't know this to start developing the
safer habit, which is always to use "is" and "is not" with None (or,
generally, with other singletons).

Hmmm... To make my code safer, I'm thinking I should replace doc strings
that say "if bluf is Null" with "if blurf compares equal to Null".

Humour?

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd www.holdenweb.com
Love me, love my blog holdenweb.blogspot.com

[Peter Hansen]

Cyril Bazin wrote:

I agree. None is an object! If you want to compare an object to another
object why not using "=="?

The term "compare" is ambiguous. If you want to compare for *identity*,
you use "is". If you want to compare for equality, you use "==". In
most cases with None you are interested in identity, not equality. It's
that simple.

As I said, the archives have extensive discussions about this with lots
of the nuances already covered in far more detail than I'm going to do
again in this thread. (The advice was free, so you're quite welcome to
consider it to be worth what you paid.)

-Peter